An audio amplifier that operates from a single power supply is shown in FIG. 1A. The audio amplifier consists of a first operation amplifier (Amp1), a second operational amplifier (Amp2), a input coupling capacitor (CIN), a first resistor R1, a second resistor (R2), and third resistor (R3), a fourth resistor (R4), a fifth resistor (R5), a sixth resistor (R6), and a reference capacitor (CR).
Resistors R5 and R6 are series connected between VHI and GND, forming a resistor divider that produces a reference voltage (VREF). A capacitor CR is connected between VREF and GND, in parallel with resistor R6, to maintain the reference voltage (VREF). The reference voltage is coupled to the non-inverting inputs of amplifiers Amp1 and Amp2.
Amplifier Amp1 produces a first output signal at node OUT−, while amplifier Amp2 produces a second output signal at node OUT+. Amplifier Amp1 is configured as an inverting amplifier, with resistor R2 connected between node V1 and OUT−, and resistor R1 connected between node V1 and VIN. Amplifier Amp2 is configured as an inverting amplifier, with resistor R4 connected between node V2 and OUT+, and resistor R3 connected between node V2 and OUT−. The two amplifiers are arranged in a bridge-amplifier configuration, providing a differential output signal (OUT+, OUT−) across a load such as a speaker (SPK).
The reference voltage (VREF) is necessary to bias the amplifiers in their optimal common mode range, such that a maximum voltage swing is available between the amplifiers output nodes OUT+ and OUT−. Since the amplifiers are biased into their optimal performance range by the reference voltage (VREF), it is necessary isolate the DC level of the input signal from the amplifier. Thus, an ac coupling capacitor (CAC) is connected between the input signal VSIG and the input of the amplifier (VIN).
Before power is applied to the circuit, capacitors CR and CIN have no charge across them (VREF and VIN are at ground). During power up, these capacitors will begin to charge at varying rates, dependent upon the various factors including the input impedance of the amplifiers, the values of the resistors and the size of the input signal VSIG. Since the input signal (VSIG) is varying in time, the differential output of the audio amplifier (OUT+, OUT−) may swing back and forth, crashing against the power supply rails, until the capacitors have charged to their respective final DC values determined by the reference voltage. Once the capacitors have achieved their “steady-state” values, the differential output of the amplifier will operate as a proper audio amplifier. The transient period during power up, where the capacitors are charging and discharging at unequal rates, results in the speaker output producing clicks and pops.
Speakers tend to have very low impedances (on the order of 4 ohms) and require current to drive the speakers. In a single power supply system, bridge amplifiers produce twice the output voltage as compared to single-ended amplifiers. Since power is proportional to the square of the output voltage, twice the output voltage swing corresponds to an increase in power output by a factor of 4. Bridge amplifiers are typically employed to provide higher power output from a single power supply.
A headphone amplifier is shown in FIG. 1B. A phono-plug type jack is used to connect a headphone set (HPA, HPB) to a dual channel amplifier. One channel (A) of the amplifier drives the left headphone (HPA), while another channel (B) drives the right headphone (HPB). Conventional jacks that are used for headphone amplifiers have a shield ring that is connected to a circuit ground, a first connection for the left channel headphone, and a second connection for right channel headphone. A first capacitor couples the output of the channel A amplifier to the left channel headphone (HPA). A second capacitor couples the output of the channel B amplifier to the right headphone (HPB).
Headphones typically have an impedance of 32 ohms, and operate at power levels on the order of 75 mW, which is considerably lower than that required by speakers. Since the output of the amplifiers drive into the headphones with a common ground (GND) connection, it is necessary to couple the output of the amplifiers to the headphones through capacitors. The first and second capacitors prevent loading down the output of the audio amplifiers with a DC load. The coupling capacitors have high values such as, for example, 100 uF.